1. Technical Field
The present invention relates to methods and apparatus for ensuring safe operation of a system in general, and methods and apparatus that utilize a safety chain circuit to ensure safe operation of, for example, an elevator system in particular.
2. Background Information
Typical elevator systems include a plurality of protection devices (e.g., door lock contacts, emergency switches, overspeed devices, etc.) connected in series. This series of protection devices is commonly referred to as a safety chain because if any of the devices (i.e. links in the chain) fail—the chain is broken and the elevator system stops operating in its normal mode. The safety chain is typically supplied with voltage from either an AC or DC source, and in some instances also includes one or more safety relays attached to the end of the safety chain. Typical safety chains are required to run the entire length of an elevator hoistway, and in many instances they run the length of the hoistway several times. As a result, the length of the transmission path of the safety chain can become prohibitively long, necessitating a power supply that is sufficiently sized to accommodate multiple, and sometimes large, relay configurations.
When contacts of each of the protection devices and relays within the safety chain are closed, the safety chain permits the normal operation of the elevator. When the contact of one or more of the protection devices or relays within the safety chain is open, an elevator brake is actuated (e.g., dropped) and power is disconnected from an elevator drive motor in order to prevent normal operation of the elevator system.